This invention relates to an image transfer system in electrostatography in which copy sheet transfer charge leakage is automatically compensated for by the transfer system.
In many electrostatographic apparatuses, a developed electrostatic latent image is transferred from an imaging surface onto a copy sheet by a transfer arrangement which comprises a transfer corotron operated at a polarity and potential such as to assist the transfer of the developed image.
A problem with transfer systems of this kind is that in order to achieve a good transfer, the paper copy sheet has to be of relatively low conductivity. If the conductivity of the paper is too high, charges on the paper immediately leak away via those parts of the machine which are in contact with the sheet, such as paper guides. Many papers suitable for xerographic copy paper have a conductivity which is satisfactory when the paper is dry, but which becomes too high for good transfer efficiency when the paper is damp. Thus, under conditions of high ambient relative humidity, many copy papers become too conductive to work properly, and in extreme cases receive virtually no transferred image at all during the transfer step of the copy cycle.
One form of transfer charge control system is described in UK Patent Specification No. 2 127 348A, in which a predetermined potential approximating the surface potential of a copy sheet during transfer is applied to a conductive guide member. The guide member acts both as a spatial limiter for the corona discharge produced by the transfer corotron and as a guide for directing copy sheets into contact with the imaging surface. The predetermined potential is supplied by a tapping of the high voltage supply for the corotrons in the apparatus, or by a separate supply.
U.S. Pat. No. 4 077 709 describes a transfer charge control system in which conductive sheet guards are electrically isolated and biassed to the same potential as the shield of the transfer corotron by means of an applied potential which is substantially ground potential.
An alternative transfer charge control system is described in U.S. Pat. No. 4 055 380, in which conductive guide members are connected to ground only through a high resistance, which provides a self-biasing floating potential on the guide members. The potential on the guide members and the potential on the shield of the transfer corotron are not directly related to one another.